Journal of Materials Chemistry b904175n
APPLICATION
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Self-assembled monolayer-assisted mass spectrometry Matthieu Bounichou, Olivier Alev^eque, Tony Breton, Marylene Dias, Lionel Sanguinet, Eric Levillain* and David Rondeau*
This article deals with the use of self-assembled monolayers (SAMs) for the formation and characterization of gaseous ions in mass spectrometry.
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Self-assembled monolayer-assisted mass spectrometry
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Matthieu Bounichou, Olivier Al ev^ eque, Tony Breton, Maryl ene Dias, Lionel Sanguinet, Eric Levillain * and David Rondeau *
Received 27th February 2009, Accepted 10th June 2009 First published as an Advance Article on the web ?????
DOI: 10.1039/b904175n
This article deals with the use of self-assembled monolayers (SAMs) for the formation and
characterization of gaseous ions in mass spectrometry (MS). The first part reviews the different results reported in the literature concerning the use of SAMs for surface induced dissociation (SID) of produced ions into a mass spectrometer. In SID/MS, the ion collision at a given kinetic energy allows the accumulation of internal energy for reaching the activation energy of fragmentation reactions. The different chemical structures of SAMs in SID/MS are described and their influence on the amount of the kinetic energy (Ekin) converted into internal energies (Eint) is reported. The second part is dedicated to the implication of SAMs in the laser desorption–ionization (LDI) methods allowing gas-phase ion formation and highlights the specifications required for the SAMs elaboration in the LDI/MS application field. The matrix-free LDI method is more particularly described. The results obtained with the so-called DIAMS technique (desorption–ionization on self-assembled monolayer surface) are reported and the organization and stability of SAMs are pointed out to obtain reliable results in LDI/
MS.
1. Introduction
The use of modified surfaces in analysis methods including mass spectrometry has showed a remarkable growth. The goal of surface functionalization is to prepare a probe that plays an active role in the purification, extraction or modification of an analyte present in a complex mixture. The organic or bioorganic surface acts then as a solid-phase extractor eitherviachemical or biochemical modification of the probe or due to inherent prop- erties of the surface. In the case of complex biological samples,
the surfaces are modified in order to selectively retain some biomolecules (peptides, proteins.) through either broad binding characteristics or very specific interactions.1,2The detection and characterization of retained compounds, by the so-called affinity array, mainly involves the use of mass spectrometry associated to a matrix-assisted laser desorption ionization (MALDI) method.3 In this framework, some original methods have been devel- oped. They involve the deposit of a matrix solution onto a specific surface which, after crystallization, promotes the desorption–ionization process of samples by absorbing and dissipating the laser beam energy.3 This concept was initially developed in 1993 by Hutchen and Yip as the surface enhanced laser desorption–ionization (SELDI) affinity technology.4,5 However, poor surface coverage caused an analyte to be non- Universite d’Angers - CNRS, Laboratoire CIMA, 2 boulevard Lavoisier
49045, Angers cedex, France. E-mail: [email protected];
[email protected]; Fax: +33 241735405
Olivier Alev^eque was born in Saint-Vallier (France) in 1979.
He studied material sciences at the Ecole Superieure d’Ingenieur de Recherche en Materiaux of the University of Bourgogne. In 2004, he joined the Laboratory of Chemistry and Molecular Engineering at the University of Angers. He is working on his PhD in physical chemistry under the direction of Eric Levillain.
His research is focused on elec- trocatalytic properties of nitroxyl radical self-assembled monolayers.
Tony Breton was born in La Rochelle (France) in 1977. He is assistant professor and works at the University of Angers (France). He received his PhD in electrochemistry under the direction of professors Jean- Michel Leger and Mustapha Belgsir in 2004 from the University of Poitiers, France.
After a 1 year postdoctoral research study on the electro- chemical modification of elec- trodes with Professor Daniel Belanger (Montreal - Canada), his current work concerns the elaboration and characterization of modified surfaces with redox centers.
APPLICATION www.rsc.org/materials | Journal of Materials Chemistry
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specifically bound to the exposed substrate, probably due to undesirable interactions (hydrophobic, van der Waals, electro- static.). In order to more precisely control the chemical and physical properties of the organic surface, SAMs have been used.6SAMs are defined as a two-dimensional film, consisting of a single layer of molecules covalently assembled at an interface.7 These molecules have to be designed to organize themselves spontaneously into crystalline or semicrystalline structures (namely by van der Waals interactions, hydrogen bonds.) and should present a function, called a headgroup, which is able to react specifically with the metal, metal oxide or semiconductor surface. In this framework, Mrksich and coworkers have combined SAMs with matrix-assisted laser desorption–ioniza- tion mass spectrometry in order to propose SAMDI (self-assembled monolayer desorption–ionization) mass spec- trometry. This method provides a powerful tool for the detection
and characterization of selective interactions, chemical or biochemical reactions at an organic surface.8–11 The use of MALDI as well as SAMDI techniques to characterize the SAMs can be also included among the numerous analytical methods used to characterize molecular structures of organic surfaces.12,13 Indeed, MS is well-known to provide direct information on the structure of a monolayer through the mass to charge ratio (m/z) measurement of the peak corresponding to the molecular constituent.14 Thermal desorption electronic impact high-reso- lution MS (TD/EI/HRMS),15secondary ion MS (SIMS)16–19and direct laser desorption–ionization MS (LDI/MS)20–22have been applied to characterize SAMs on gold.
SAMs for mass spectrometry appear to be less developed and only two main utilizations have been reported in the literature.
The first is the use of SAMs as a matrix-free laser desorption–
ionization (LDI) technique to produced ions in the gas-phase
Marylene Dias was born in Gourdon (France) in 1967. She studied chemistry at the Paul Sabatier University of Toulouse and at the University of Angers.
She received her PhD degree in organic chemistry from the University of Angers in 1994 under the supervision of Professor Rene Mornet. After a postdoctoral stay with Professor J. Grimshaw at Queen’s University of Belfast, she became assistant professor in 1996 at the University of Angers. Her research interests are organic and bioorganic chem- istry and her current work concerns the elaboration of target molecular materials dedicated to Laser Desorption/Ionization (LDI) processes.
Eric Levillain was born in Rouen (France) in 1964. He studied chemistry at Hautes Etudes Industrielles (HEI – Lille) and at the University of Lille. He received his PhD degree in physical chemistry from the University of Lille in 1992 under the supervision of Dr Jean- Pierre Lelieur. He became Charge de Recherche CNRS in 1993 at the University of Lille, then Directeur de Recherche CNRS in 2003 at the University of Angers. His research interests include electrochemistry, spectroelectrochemistry and redox- responsive materials with special emphasis on self-assembled monolayers. He is the author or co-author of over 120 research articles.
Lionel Sanguinet was born in Bordeaux (France) in 1975. He studied chemistry at the University of Bordeaux I where he received his PhD degree in organic chemistry in 2003 under the supervision of Professors J.- L. Pozzo and V. Rodriguez.
After a postdoctoral stay with Professor R. J. Twieg at the Kent State University (U.S.A), he joined the laboratoire de Chimie et d’Ingenierie Molecu- laire d’Angers in 2005 as a post- doctoral fellow and was promoted to assistant professor in 2007. His research interests mainly concern the elaboration and characterization of molecular materials incorporating electroactive or photoactive molecular receptors.
David Rondeau was born in Saint-Brieuc (France) in 1968.
He studied biochemistry at first then chemistry in the University of Rennes 1 where he obtained his PhD degree in chemistry (synthesis and electrochem- istry) under the direction of Professor Andre Tallec. After three years in industry as a researcher in analytical chem- istry, he joined the University of Angers as an engineer in the Spectroscopic Analysis Service.
After an assistant professor position at Angers, he is now professor at the University of Bre- tagne Occidentale (Brest). His research is focused on the chem- ical-physics processes involved in mass spectrometry and their application in analytical chemistry.
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prior to their mass analysis via a time of flight (TOF) mass spectrometer. The second is the use of SAMs as an ion charac- terization method by surface induced dissociation (SID) mass spectrometry.
SAMs for SID-MS and for matrix-free LDI/MS are the two approaches described in this paper which deal with the use of SAMs for mass spectrometry.
2. SAMs for gaseous ion characterization
In mass spectrometry, the identification of compounds is per- formed both by mass to charge ratio measurements and by fragmentation of the ionic species of interest. Experiments using ion fragmentation generally use tandem mass spectrometry and are called MS/MS or MSn experiments.23 Ions are produced upstream in the ion source; the precursor ion is selected by a first analyzer for ion beam mass spectrometers or after selection sequences in the case of ion trapping devices. Ion dissociation involves the use of an activation method that increases the internal energy of the selected ion in order to reach the critical energy of fragmentation reactions.24After the decay, the frag- ments are independently characterized by their mass to charge ratio. There are numerous activation techniques available on commercial mass spectrometers. Among these, the use of a target gas (argon, helium.) leads to the process known as the collision induced dissociation (CID) where an amount of the kinetic energy (Ekin) of the incident ion is converted into the internal energy (Eint) of the precursor ion by inelastic collision.25 In a high-collision energy regime (0.1–10 keV) the high-energy CID process is more efficient if the interaction time coincides with a vertical electronic transition. The bond cleavage occurs after redistribution of the internal energy acquired into vibrational modes. In low-energy collision regimes, the interaction time is related to the ro-vibrational excitation processes and the total energy transferred to the ion at each collision is called the center of mass energy (Ecom). It depends on the ion’s kinetic energy (Elab), the mass of the neutral gas (mN) and the precursor ion (mp) as described in eqn (1) such as:
Ecom¼ mN mNþmp
Elab (1)
whereElabis also known as the laboratory energy. The amount of the internal energy deposition can be increased either by increasing the collision energy,i.e. EkinormN, or by increasing the collision number. The use of the surface instead of gas appears as being a good alternative to high-energy and low- energy CID.26,27 Cooks and coworkers showed as far back as 1985 that the SID principle can be used for structural charac- terization.28,29The surface induced dissociation (SID) method is not proposed in a commercial mass spectrometer but it has been the subject of numerous developments in laboratories.30–32One of the major contributions of the SID/MS analysis to the struc- tural characterization by mass spectrometry is that the one-step fast ion-activation method offered by the ion-surface collision process allows one to explore the energetics and mechanisms of peptide fragmentation.33–40 In SID, mass-selected ions with a given kinetic energy collide with a surface according to an incident angle Qinc (typically between 45 to 55) and the
secondary ion beam is scattered from the surface (with a scat- tering angleQscat) to be mass analyzed (see Fig. 1).41
SID experiments are often performed in the well-known hyperthermal energy regime characterizing a projectile trans- lational energy ranging between 1 to 100 eV.41There are different fundamental molecule–surface collision processes in the energy range used for incident ion fragmentation such as the soft- landing, the ion–surface reaction and the chemical sputtering processes. For all, it has been shown that the SID is a versatile method to deposit a relatively narrow distribution of internal energy and to perform dissociation reactions activated by high average internal energies.42–45
The early stages of SID development employ metallic surfaces.
However, the neutralization of a primary incident ion beam was described as the major disadvantage when a metal surface (stainless steel or gold) is used.46–48 The presence of surface contamination by physisorbed materials was also invoked to explain either the occurrence of charge transfer reactions and the observation of interfering ions on the SID MS/MS mass spectra.49Such behaviors are due to the low potential energy of the materials and the subsequent release of surface-adsorbed species by chemical sputtering.50To reduce surface neutraliza- tion, self assembled monolayer (SAM) coatings, chemically bound to metal surfaces were used. Since the pioneering work of Morriset al,51SID on SAMs have been performed using (i)n- alkanethiols of different chain lengths (CH3(CH2)nSH,n¼3, 11, 14–17)52–57 (ii) perdeuterioalkanethiol (CD3(CD2)nSH, n ¼ 15,19)51,52,57,58 (iii) 2-(perfluorooctyl)ethanethiols (CF3(CF2)n CH2CH2SH, n ¼ 7–11)52,54,57,58 and (iv) 4-(4-alkoxyphe- nyl)benzenethiols (4-(4-(CH3(CH2)nOC6H4)–C6H4–SH,n¼14– 2 17)53 as covalently bounded organic entities on gold or silver surfaces by sulfur–metal bonds. The results described in the literature show that projectile ion neutralization is less favorable for highly ordered, thick crystalline surfaces such as C12and C18 alkanethiolates and fluorinated alkanethiolates, than for gold, stainless steel and shorter chain alkanethiolates.52The decrease in neutralization with fluorocarbon SAMs (F-SAM) is due to their resistance against contamination by hydrocarbons from the pump oil and the relatively high ionization energies of the F- SAM isolated molecules, typically around 14 eV. In all cases, the Fig. 1 Principle of the SID process (QincandQscatare the incident and scattering angle, respectively).
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electron barrier properties of the films under the mass spec- trometer vacuum are described as identical to those reported for electrochemistry measurements in solution.52,59In addition, the relative organization degree of the SAMs seems to be retained upon transfer of the surface from the solution to the vacuum.52 The chemical nature of the SAM is linked to the kinetic-to- internal energy conversion (Ekin/Eint) during the SID process.
It appears from several different methods that a much higher energy conversion was observed for F-SAMs considered as ‘‘a hard wall’’ than for hydrocarbon SAMs (H-SAM) which behaved like ‘‘a soft mattress’’. The different results show that the Ekin/Eint> 20% for F-SAMs and around 15% for H-SAMs (as for stainless steel surfaces).43,45,51,55,58,60,61 The influence of the alkyl chain orientation on the nature of the ions observed on the ion-surface collision mass spectra has been disclosed with the used of odd-chain-length and even-chain-length films.53For the former, the last C–C bond is more parallel to the plane of the surface whereas the last film places the hydrogen atoms in more favorable position for reacting with the incident ion beam. Few papers highlight that SID mass spectra are also sensitive to the preparation mode of the gold surface prior to the assembly reaction (mechanically polished gold foil vs. vapor deposited gold vs. plasma-cleaned vapor-deposited) and to the exposure time of the metal surface in the SAM’s molecular precursor solutions.52On the other hand, the presence of defects on the SAM exposed to SID experiments such as pinholes in the monolayer, do not appear to be considered in the penetration of impinging ions. To the best of our knowledge, any studies were focused on the relationship between the state of the SAM and the neutralization yield of the incident ionic beam, the nature of ions observed on the SID mass spectra and the percentage of theEkin
toEintconversion.
3. SAMs for laser desorption–ionization of molecules in mass spectrometry
The gaseous ion formation from sample molecules is not commonly associated with the use of SAMs. However, SAMs can constitute a photon-absorbing mediator for the vaporization and the ionization of analytes in the case of the laser desorption–
ionization process (LDI). In mass spectrometry, the LDI method refers to an overall process by which the energy absorption of a laser beam by a localized region of an irradiated surface leads to the emission of gaseous charged particles. The surface has to participate either
3 in the rapid dissipation of the absorbed energy and to the vaporization of the analyte. Note that the molecule of the sample is either already at a charged state in the condensed phase of the spot or will be ionized by ion–molecule reaction in the laser plume near the surface. Nevertheless, all these processes have to occur instead of a pyrolysis reaction of the sample. This one is due to the direct absorption in the UV or IR region by the analyte that activates the dissociation of weak bonds during the energy transfer. This is why Karaset al.62and Tanakaet al.63 have developed the matrix assisted laser desorption–ionisation (MALDI) method where the sample is dispersed into crystals of an aromatic matrix molecule that absorbs at the photon energy laser wavelength.64,65 In this case, the irradiated matrix crystal surface plays a key role in the energy dissipation process, leading to the vaporization of the matrix and the release of the analyte
molecules by translational excitation. In the MALDI experi- ments, the detected charged species are either preformed before being desorbed or produced in the laser plume after vaporization of neutrals that undergo gas-phase ionization by a matrix–ana- lyte reaction.66The use of such ‘‘softened’’ ablation–ionization processes have allowed one to consider MALDI as a powerful tool for the formation of characteristic gaseous ions from high molecular weight molecules (MW > 5000 u).67The main disad- vantage of the MALDI process is that it cannot be employed for the analysis of low molecular weight compounds (MW < 700 u) or for high-throughput analyses. The UV irradiation of the matrix solution produces a wide range of ions in the low-mass range because the matrix molecule, a fairly low molecular weight, is the major species (i.e.with a molar ratio of 1000 to 10000 by regards of the analyte). In order to overcome these limitations, some matrix-free LDI methods were proposed in the literature.
The thin gold film-assisted laser desorption–ionization (TGFA- LDI) technique was developed by Wahlet al.68The TGFA-LDI method uses IR (1064 nm) or visible (532 nm) beams for laser desorption–ionization of organic samples deposited onto a gold surface; [M + Na]+and [M + K]+ions were then produced from gramicidine S. The team of Siuzdak and coworkers developed the laser desorption–ionization on porous silicon (DIOS) method with regards to the structure of porous silicon that provides a scaffold for retaining solvent and analyte molecules, and the UV absorption that affords a mechanism for the transfer of the laser energy to the analyte.69The aim is to limit the direct heating of a sample by keeping the absorptionand dissipation energy of 4 the surface. DIOS mass spectrometry has allowed the detection of protonated molecules and alkali metal adducts from peptides and organic compounds.70Considering the surface properties of a UV-absorbing semiconductor like porous silicon and the analytical flexibility offered by organic functionalized surfaces, the use of SAMs in order to develop new LDI techniques is promising. The first examples of such methods, named desorp- tion–ionization on self-assembled monolayer surfaces (DIAMS) were published recently.71
3.1 Specifications required for the SAMs elaboration
The concept of the so-called DIAMS (desorption–ionization on self-assembled monolayer surfaces) method is illustrated in Fig. 2.
Specific parameters must be controlled to employ SAMs in laser desorption–ionization. First of all, the molecular precursor has to lead to a stable bond with the metallic surface for resisting a potential warm-up of the metal–organic monolayer interface (see part a in the Fig. 2). Second, the functional moiety of the molecule selected to be grafted on the surface (see part c in the Fig. 2) has to absorb at the laser wavelength. Finally, the molecular design should drivein order to maintain the conduc- 5 tivity surface properties and help the organization of the mono- layer. For these reasons, the monolayer has to be reinforced by the spacer that separates the headgroup of the molecular precursor to its functional moiety (see part b in the Fig. 2). In the DIAMS method, the molecular design could be resumed as an alkanethiol covalently bound to a redox chromophore which has been grafted on a gold surface (see Scheme 1).
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The S–Au bond is known for its easy elaboration and chemical stability.72Note also that a wide range of studies demonstrate the slow degradation process of the organic monolayers covalently fixed to the gold surface by the way of a sulfur moiety. Such devices account for reproducibility studies of so-prepared DIAMS plates.73–75The spacer, which plays a key role to achieve the 2D self-assembly of the monolayer, was selected to be long enough to authorize a high density of alkyl–alkyl interactions and consequently increase the stability of the layer.76However, to conserve surface conductivity properties, the alkyl chain has to not exceed a maximum length. As a result, a spacer constituting of ten carbons (n ¼ 6 in Fig. 2) was preferred for the first development of the DIAMS method. Regarding the functional moiety, the attention was focused on a 5,50-disubstitued-2,20- bisthiophene unit (see Scheme 1). In fact, this unit presents very promising optical properties (its molar extinction coefficient maximum is near the laser wavelength used in DIAMS experi- ments,i.e.337 nm) and redox properties (a reversible one elec- tron process in a positive direction allows the non-destructive
characterization of the functionalized surface).77 However, the planar characteristics of the 5,50-disubstitued-2,20-bisthiophene unit would not greatly influence the monolayer organization initiated by the nature and the length of the spacer.
The hydrophilic/hydrophobic character can also be tuned with mixed SAMs.78 Shorter spacers or modifications of the func- tional moiety would also bring some additional chemical or physical properties to the functionalized surface and mixed SAMs could induce a decrease of the chromophore coverage.
Therefore, the choice of a functional moiety is decisive to enhance the efficient deposition.
3.2 The use of SAMs in mass spectrometry as a new matrix- free LDI/MS method
The range of samples experienced by DIAMS/MS encompasses compounds as different as peptides and glycerides in positive ion mode and fatty acids in negative ion mode.71,79 The DIAMS method has been compared with the usual LDI techniques, through the analysis of a mixture of five glycerides in DIAMS/
MS, MALDI/MS and TGFA-LDI/MS.79 It was shown that DIAMS/MS requires a lower energy laser than MALDI and 6 TGFA-LDI methods in order to observe analyte ions in mass spectra.
The DIAMS method was also assessed with and without NaI because the use of a cationic agent, such as sodium iodide, is often used to improve the detection sensitivity for the glyceride analysis in MALDI-TOF18,19 or LDI,21 by the way of adduct formation ([M + Na]+, [M + K]+). The results were then compared with TGFA-LDI and MALDI methods, using the same experimental conditions. From this study, it was demon- strated that DIAMS/MS is as statistically repeatable and reproducible as MALDI/MS and TGFA-LDI/MS. A compa- rable repeatability/reproducibility has been estimated at 15% and 30% and the relative detection limit was evaluated at 0.3 pmol and 3 pmol, with and without NaI respectively. In addition, the possibility of obtaining quasi-molecular [M-H] ions from low molecular weight compounds such as the salicylic acid and itsd6
isomer has confirmed the additional interest of DIAMS in the detection and quantification of small molecules.
Now, as recalled in the case of SID experiments using SAMs, we must keep in mind that the DIAMS/MS method allows one to perform repeatable and reproducible measurements only if the monolayers are properly elaborated. Indeed, it appears that if the cyclic voltammetry of the SAMs does not show a reversible redox system characteristic of a stable monolayer confined at the metallic surface, the quality of the mass spectra obtained dramatically decreases.79 We assume that, in this case, the monolayer is not homogeneous and must present some impor- tant defects. This fact involves a decrease of the surface coverage, as well as a lower organization of the layer, leading to the loss of the SAM’s stability.
4. Conclusion
Relating to the ionization (matrix-free LDI/MS) and fragmen- tation (SID/MS) of some analytes, the use of SAMs is now an established and promising approach for both technologies.
However, the future explorations on self-assembled monolayer- Fig. 2 Schematic illustration of the DIAMS (desorption–ionization on
self-assembled monolayer surface) method concept for the desorption–
ionization laser of samples directly deposited onto the organic surface showing that the organic monolayer has to be (a) covalently bonded to the gold surface, (b) spontaneously organized on a crystalline or semi- crystalline structure and (c) constituted with a functional part that absorbs at the laser wavelength and maintains the conductivity properties of the metallic sample plate. Note that the black full circles represent the sample deposited onto the surface as represented. It must be noted that for the present figure, any assumption is formulated on the position of the sample on or into the monolayer, and not about the conversion modes of the photon energy to the translational energy during the DIAMS process.
Scheme 1 Structure of the so-called bisthiophene-alkanethiol used to prepare the SAMs in DIAMS/MS.
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assisted mass spectrometry have to clarify thermal and electro- chemical SAMs activation.
Because SAMs can assemble onto surfaces of any geometry or size, they provide a general and highly flexible method to tailor the interfaces between nanometre-scale structures and their environment with molecular (i.e.subnanometre scale) precision.
Indeed, SAMs can be either designed to absorb or to dissipate the energy provided by a laser beam in order to allow the vapor- ization of the analyte which acquires translational energy or, designed to favor the conversion of the kinetic energy of the flying ions into internal energy in order to induce preferentially some ion dissociation processes. SAMs are a prototypical form of nanotechnology: the molecules that form the SAM carry the
‘‘instructions’’ required to generate an ordered, nanostructured material without external intervention. SAMs demonstrate, once more, that molecular-scale design, synthesis, and organization can generate macroscopic materials properties and functions.
Acknowledgements
We gratefully acknowledge the ‘‘Centre National de la Recherche Scientifique’’ (CNRS - France) for the PhD scholarship to Matthieu Bounichou and financial support provided by the
‘‘Agence Nationale de la Recherche’’ (ANR - France) and the
‘‘Region des Pays de la Loire’’ (France).
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Title:Self-assembled monolayer-assisted mass spectrometry
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